In recent years, development of a service robot used in circumstances in which the robot exists in proximity to a person has been promoted. For example, a bipedal walking robot is known, which does some performance at entertainment venues or walks among people. Furthermore, research of care support robots has been advancing to put them into practical use.
When a robot and a person exist closely with each other, it is important to take safety measures assuming a case in which the person contacts the robot.
For example, when a robot and a person contact with each other, it is required to mitigate the force acted at the time of contact.
In order to achieve this, it is required to give so-called softness to joints of a robot.
Service robots are required to have light weight, high torque capability, and perform operation control with high accuracy. From these points of views, the combination of a servo motor and a differential speed reducer has been adopted in the joint unit of almost all the service robots. The differential speed reducer has compactness, light weight, high accuracy, and high reduction ratios, and is the element indispensable for driving joints of a service robot. While such a differential speed reducer is able to achieve driving with high accuracy with no backlash in a compact space, its back drivability is extremely low. In particular, in a state in which the joints are stopped, a large static friction acts on mesh of gears in the differential speed reducer. Accordingly, unless extremely large torque is applied from an output side, it is impossible to rotate the joint from the output side against the static friction in the differential speed reducer.
There is a method of dithering.
It means oscillating the motor by a high-frequency wave with small amplitude in a joint stationary state, thereby artificially producing a state in which the static friction does not act.
A friction model is disclosed in a non-patent literature 1, for example. FIG. 11 shows this friction model. Although there are many other friction models, the basic pattern is substantially the same.
As will be understood from this friction model, large friction (static friction) acts in a low-speed region. Then, after the friction is reduced once to a certain speed, the friction force (viscous friction) increases along an increase in the speed. This is the typical friction model. In a non-patent literature 2, it is examined how much the friction at the time of operation start is reduced by dithering. In the non-patent literature 2, it is demonstrated that the external force required at the time of operation start can be made smaller when dithering is performed, and it is reported that back drivability may be improved by using dithering. A method of reducing the friction by such dithering is applied in the patent literature 1, for example.